By setting CLUSTER_DATABASE=FALSE, you are configuring your system to be a single-node database. This in turn enables you to 'MOUNT EXCLUSIVE,' allowing you to switch the archive mode for the instance. As you indicated, you need to set this parameter value back to TRUE to re-establish your RAC configuration after the archiving mode has been changed.
In 10gr2, you need not change this parameter to accomplish this task; you merely have to make sure the instance you are working from is the only instance with the database mounted.
Also, it is very important to take a fresh FULL backup of your database after changing from noarchivelog to archivelog. You will need this backup as the starting point to apply your archive logs in case of recovery.